NEW CONTRIBUTIONS TO THE VERTEBRATE ASSEMBLAGE OF THE UPPER CRETACEOUS (CAMPANIAN) NESLEN FORMATION, UTAH, USA

The Western Interior of North America preserves numerous vertebrate-bearing fossil assemblages in fluvial basins from the Campanian stage of the Upper Cretaceous, stretching from Mexico to Alberta. These penecontemporaneous assemblages have served as a study system for understanding paleobiogeography and evolutionary patterns during the Late Cretaceous, supporting hypotheses of provinciality and endemism. I present new contributions to the vertebrate fossil assemblage from the Campanian (~75 Ma) Neslen Formation of east-central Utah. The Neslen Fm represents deposition in fluvial to paralic settings in the Sevier foreland basin. Compared to units of similar age, the Neslen Fm has not been surveyed as much for its paleontological resources. Thus far, only two vertebrates – the hadrosaur Rhinorex and a partial tyrannosaur hindlimb – have been published from the formation.

Two field seasons in the Neslen Fm have produced over 100 new cataloged specimens. The lower portion of the formation, especially the Palisades Coal Zone, is the most fossiliferous. Specimens collected include those of angiosperms, gymnosperms, freshwater molluscs, turtles, crocodilians, and dinosaurs. Though most of the vertebrate material consists of isolated elements, many have proven to be diagnostic. Turtles identified from the formation include Adocus, Basilemys, Neurankylus, a plastomenine trionychid (c.f. Helopanoplia), and two other trionychids. New dinosaurs from the formation include dromaeosaur and tyrannosaur teeth, hadrosaurs, ankylosaur osteoderms, and ceratopsian elements. One ceratopsian specimen is represented by a postorbital horn preserving a portion of the orbital margin. That specimen exhibits a morphology not observed in named ceratopsians.

Preliminary results support the presence of components of both the hypothesized northern and southern Campanian biogeographic provinces. Plastomenines, including the genus Helopanoplia, are more common to the south (southern Utah and New Mexico). The published tyrannosaur from the formation possesses features most similar to northern (Montana and Alberta) tyrannosaurs. Continued work on the vertebrate assemblage of the Neslen Fm will further elucidate the nature of latitudinal paleobiogeographic patterns across the Cretaceous Western Interior.